Sugar has been getting a lot of bad press recently, but it's surprisingly
useful – in the garden

Given thecurrent fuss about dietary sugar, you may have a cupboard full of the stuff that you are too frightened to eat. If so, one solution might be to tip it onto your plants.

In the average garden, the biggest barrier to establishing an interesting wildflower meadow is high soil fertility. This may sound counterintuitive, but in fertile soil other plants – particularly grasses – outcompete the wildflowers.

The quick, but drastic, solution is to remove the topsoil and start again with the subsoil. Easier, but slower, is to cut and remove the vegetation every year. Sowing parasitic yellow rattle, which targets coarse grasses, will help, but not unless high levels of soil fertility can be reduced first.

But there is another, slightly surprising, fix. The plants in your garden compete for nutrients not only with each other but also with an army of microbes. Normally this competition is a bit one-sided, because the plants have a ready supply of carbon (from photosynthesis) that allows them to grow lots of roots.

Microbes, in contrast, have no independent supply of carbon and have to make do with decomposing organic matter, plus whatever they can borrow or steal from the plants. This is never as much as they would like, so they are permanently undernourished.

You can level the playing field by supplying extra carbon, and the simplest way to do this is to add sugar. This gives soil microbes a shot in the arm, so they grow faster and compete more effectively with the plants for nutrients. The total amount of soil nutrients doesn’t change, but there is less for the plants, so these grow more slowly and biomass goes down.

We have known this for a long time, but I always worried that the effect might be short-lived. A study from Estonia, however, published in the Journal of Vegetation Science, shows that it isn’t. The researchers added sugar every year to a hay meadow. After 10 years the weight of vegetation was lower and the amount of bare ground greater in plots treated with sugar than in control plots.

They also added fertiliser to other plots, which had a predictable effect: biomass went up and diversity went down. The big winner in fertilised plots was our old friend couch grass. The effect of sugar on diversity, however, was disappointing. Despite the lower biomass, diversity didn’t change much, and after 10 years there were only subtle differences between control plots and those treated with sugar.

To try to find out why, they added seeds of the common plantain, Plantago lanceolata. Again predictably, none of the plantain seedlings in the fertilised plots survived. This is one of the main reasons diversity is so low in grasslands on fertile soils: competition from established plants prevents any new plants establishing from seed. Some seedlings survived in both control and sugar-treated plots, but more survived in the latter.

What the plantain results show is that the failure of diversity to increase naturally in these plots is most likely a dispersal problem; there simply aren’t many other species around to take advantage.

In a garden, of course, you could intervene on a more ambitious scale and add something more interesting and attractive than plantains. But they illustrate the key principle: if you want to brighten up some dull grassland by sowing wildflowers, adding sugar gives them a fighting chance; without it, they will probably fail. You should also, of course, cut and remove the hay every year, traditionally in late summer.

In case you’re wondering, the researchers added 1kg of sugar per square metre every year, at the end of May and the beginning of September (500g each time). The effect not only persists as long as sugar is added but also happens very quickly – if you add sugar in spring, plant growth will already be reduced by the summer.

*Ken Thompson is a plant biologist with a keen interest in the science of gardening. He writes and lectures extensively, and has written four gardening books. His latest book is Where do Camels Belong? The Story and Science of Invasive Species